1. Field of the Invention
The present invention generally relates to a radio communications apparatus that uses an array antenna, and especially relates to a calibration method of a phase difference and a gain ratio (amplitude difference) at an antenna edge.
2. Description of the Related Art
Development of digital cellular radio communications systems using DS-CDMA (Direct Spread Code Division Multiple Access) technology as the next-generation mobile communications system is progressing. The CDMA method defines a channel by assigning a code unique to each channel and offers simultaneous multi-channel communications. Here, communications of a channel are often interfered with by signals of other channels. Consequently the number of channels that can be offered, i.e., channel capacity, is limited. Conversely, the channel capacity can be increased if the interference is effectively suppressed.
An adaptive array antenna adaptively forms a beam for a user of choice according to the environment such that a null point is formed toward a source of great interference, enabling an increase in the channel capacity. That is, a beam is formed in the direction of the user of choice such that a radio wave from the user of choice is received with sufficient sensitivity, and a radio wave from the source of great interference is suppressed by setting a null point in the direction of the source of the great interference. In this manner, the amount of interference can be reduced, and the channel capacity can be increased.
Here, the adaptive array antenna provides a beam by using a phase difference and an amplitude difference at an antenna edge. For this reason, if the phase and the amplitude (gain) fluctuate at each radio frequency signal receiving/transmitting unit, adequate control of the beam pattern becomes unavailable.
Therefore, in order to adequately control the beam pattern, it is necessary to compensate for the gain and amplitude differences at each antenna edge. The differences of the phase and the gain are compensated for by multiplexing a calibration signal, and by detecting and compensating for differences of the phase and the gain of the multiplexed signal.
FIG. 23 shows a conventional calibration apparatus for array antennas (Patent Reference 1). According to the conventional calibration apparatus, the calibration signal is spread spectrum in the radio bandwidth by a spreading unit 309. For this reason, in order to demodulate the spectrum spreading signal, it is necessary to perform a despreading process, and to detect a correlation value of each radio frequency signal receiving unit. The despreading process is performed based on coherent timing detected by a coherent unit 303 that detects the coherent timing of the calibration signal received by each radio frequency signal receiving unit.
[Patent Reference 1] JP, 11-46180, A
[Problem(s) to be Solved by the Invention]
As described above, according to the conventional calibration apparatus, in order to restore the calibration signal, the despreading process is performed and the correlation value is detected for each radio frequency signal receiving unit. For this reason, the function to detect coherent timing is indispensable. Further, even if coherent timing is input from a calibration signal generating unit, in the case that timing of the calibration signal generating unit is in units of chips and timing of the radio frequency signal receiving unit follows a 4-time over sampling technique, precision of ¼ chip is needed. For this reason, adjustment in units of quarters of a chip is indispensable.
That is, the conventional calibration apparatus needs to detect the coherent timing, for example, at ¼-chip precision. On the other hand, as for calibration, what is important is to obtain differences of the phase and the gain between the antennas, and detection of coherent timing is theoretically unnecessary.